1.3.3. How Certificates Are Used

Certificates have a purpose: to establish trust. Their usage varies depending on the kind of trust they are used to ensure. Some kinds of certificates are used to verify the identity of the presenter; others are used to verify that an object or item has not been tampered with.

1.3.3.1. Uses for Certificates

1.3.3.1.1. SSL
The Secure Sockets Layer (SSL) protocol governs server authentication, client authentication, and encrypted communication between servers and clients. SSL is widely used on the Internet, especially for interactions that involve exchanging confidential information such as credit card numbers.
SSL requires an SSL server certificate. As part of the initial SSL handshake, the server presents its certificate to the client to authenticate the server's identity. The authentication uses public-key encryption and digital signatures to confirm that the server is the server it claims to be. Once the server has been authenticated, the client and server use symmetric-key encryption, which is very fast, to encrypt all the information exchanged for the remainder of the session and to detect any tampering.
Servers may be configured to require client authentication as well as server authentication. In this case, after server authentication is successfully completed, the client must also present its certificate to the server to authenticate the client's identity before the encrypted SSL session can be established.
For an overview of client authentication over SSL and how it differs from password-based authentication, see Section 1.3.2, “Authentication Confirms an Identity”.
1.3.3.1.2. Signed and Encrypted Email
Some email programs support digitally signed and encrypted email using a widely accepted protocol known as Secure Multipurpose Internet Mail Extension (S/MIME). Using S/MIME to sign or encrypt email messages requires the sender of the message to have an S/MIME certificate.
An email message that includes a digital signature provides some assurance that it was sent by the person whose name appears in the message header, thus authenticating the sender. If the digital signature cannot be validated by the email software, the user is alerted.
The digital signature is unique to the message it accompanies. If the message received differs in any way from the message that was sent, even by adding or deleting a single character, the digital signature cannot be validated. Therefore, signed email also provides assurance that the email has not been tampered with. This kind of assurance is known as nonrepudiation, which makes it difficult for the sender to deny having sent the message. This is important for business communication. For information about the way digital signatures work, see Section 1.2, “Digital Signatures”.
S/MIME also makes it possible to encrypt email messages, which is important for some business users. However, using encryption for email requires careful planning. If the recipient of encrypted email messages loses the private key and does not have access to a backup copy of the key, the encrypted messages can never be decrypted.
1.3.3.1.3. Single Sign-on
Network users are frequently required to remember multiple passwords for the various services they use. For example, a user might have to type a different password to log into the network, collect email, use directory services, use the corporate calendar program, and access various servers. Multiple passwords are an ongoing headache for both users and system administrators. Users have difficulty keeping track of different passwords, tend to choose poor ones, and tend to write them down in obvious places. Administrators must keep track of a separate password database on each server and deal with potential security problems related to the fact that passwords are sent over the network routinely and frequently.
Solving this problem requires some way for a user to log in once, using a single password, and get authenticated access to all network resources that user is authorized to use-without sending any passwords over the network. This capability is known as single sign-on.
Both client SSL certificates and S/MIME certificates can play a significant role in a comprehensive single sign-on solution. For example, one form of single sign-on supported by Red Hat products relies on SSL client authentication. A user can log in once, using a single password to the local client's private-key database, and get authenticated access to all SSL-enabled servers that user is authorized to use-without sending any passwords over the network. This approach simplifies access for users, because they don't need to enter passwords for each new server. It also simplifies network management, since administrators can control access by controlling lists of certificate authorities (CAs) rather than much longer lists of users and passwords.
In addition to using certificates, a complete single-sign on solution must address the need to interoperate with enterprise systems, such as the underlying operating system, that rely on passwords or other forms of authentication.
1.3.3.1.4. Object Signing
Many software technologies support a set of tools called object signing. Object signing uses standard techniques of public-key cryptography to let users get reliable information about code they download in much the same way they can get reliable information about shrink-wrapped software.
Most important, object signing helps users and network administrators implement decisions about software distributed over intranets or the Internet-for example, whether to allow Java applets signed by a given entity to use specific computer capabilities on specific users' machines.
The objects signed with object signing technology can be applets or other Java code, JavaScript scripts, plug-ins, or any kind of file. The signature is a digital signature. Signed objects and their signatures are typically stored in a special file called a JAR file.
Software developers and others who wish to sign files using object-signing technology must first obtain an object-signing certificate.